The present invention is a semi-automated device for thinning young tree plantations to achieve proper stocking levels or for selectively falling larger trees during thinning or final harvesting operations.
The heavy need for lumber, pulp and other forest products has led to entirely new ideas of forest management in the last three decades. In this time the timber industry has turned from a log supply harvested from wild forests to a current practice of intensively managed tree plantations. A typical crop cycle may vary between 25 and 100 years, depending on many factors. Among these are tree species, soil productivity and climate. The economic benefits of keeping this crop cycle as short as possible are fully obvious. To further this end, forest managers have turned to such practices as the planting of genetically improved seedlings, fertilization at one or more times during the growth cycle and periodic thinning of the crop as the trees grow larger and begin to compete for space and nutrients.
Silvicultural practices vary between different areas of the country because of terrain, climate and tree species variation. in the southeastern United States and in other parts of the world where the terrain is of flat to moderate relief, much more mechanization is possible than in more mountainous forest areas. In many southern pine areas trees can be planted in relatively straight, parallel rows with the assistance of tractor-drawn equipment. These stands are inherently more tractable for mechanized silvicultural practices than are randomly planted stands.
Cutover land is normally restocked with seedlings at much higher densities than is desirable in the final years before harvest. There are a number of reasons for this. Mortality is often high in the early years and overstocking is one way to accommodate this problem. Tree form is also improved when stands are somewhat more crowded in their earlier years because the tendency to develop heavy lower limbs is reduced. Overstocking also provides some natural control of competitive trees and brush in the early years of the stand.
It is common practice to restock plantations at levels of 1500 to 2500 seedlings per hectare. In the southern pine area this is frequently done using a row spacing of approximately three meters with seedlings every one to two meters along each row. By the time the plantation is six to eight years old the trees will be four to seven meters tall and have a lower stem diameter about 7 to 15 cm. At this time a precommercial thinning is usually made to reduce stocking level to about 1000 trees per hectare. By precommercial is meant that the cut trees are usually not brought out of the woods but are allowed to remain and decay. As the value of forest biomass for fuel increases it may be economical to collect these trees but this is not presently the case due to their very small size. Subsequent thinnings, when the removed stems are commercially utilized, typically reduce the stocking level to about 370 trees per hectare by 20 to 30 years of age. This level is maintained until the entire stand is ultimately harvested and the land again made ready for a new crop.
At the present, virtually all precommercial thinning is done manually by crews using chainsaws. This is slow, difficult and sometimes hazardous work. A trained cutter may be able to thin from 0.4 to 1.2 hectares per man in an eight-hour day, this variability being largely a factor of brush density on the site.
Few prior attempts have been made to mechanize this job. One example in U.S. Pat. No. 2,580,110, which shows a tractor-mounted saw could, by a far stretch of the imagination, conceivably be adopted for stocking control although the embodiment shown is apparently intended for bucking cord wood. A more practical machine for stocking control is a tractor with a saw mounted on a swinging boom that is apparently hydraulically activated, as shown in the article by C. M. Kerruish, Thinning Techniques Applicable to Pinus Radiata Plantations, New Zealand Journal of Forest Science, 6 (2), 201-2 (1976).
Other much heavier machines primarily intended for site preparation have also been considered for stocking control. One such machine is the Trakmac, made by Washington Iron Works, Seattle, Wash. This consists of an articulated double-tracked prime mover with a swinging boom in the front that carries a large and very rugged inertia cutter. In general, this and other machines are too heavy, too expensive and require a very high degree of operator skill to avoid damage to trees selected to remain standing.
Any mechanized stocking control equipment has two difficult problems to accommodate. The trees are only rarely found growing in exactly straight rows. Variations of 20 to 30 cm either side of the row axis are common even with machine-planted trees. These variations are usually even greater when the plantations have been hand planted. A second problem is that of debris on the ground, such as rocks and old stumps, that would cause a tractor driver to take a somewhat zig-zag course rather than one exactly parallel to the tree row. There are thus two error zones that must be accommodated; one around the tree row axis and another around the preferred line of tractor travel. Because of this, with a simple boom-mounted saw, such as the one shown by Kerruish, the tractor driver is going to spend a high proportion of his time steering the tractor into a position where he can cut the individual trees to be removed. This greatly slows the operation to the point where a machine of this type is only marginally advantageous, at best, over ground crews using chainsaws.